Is there any apparent advantage of using spamd/spamd-setup via cron
without actually redirecting the packets to spamd? Instead of
tarpitting
blacklisted mail relays, you could simply drop the packets to IPs
listed
in the <spamd> table with a rule such as the following:

block in quick log on $ExtIf from <spamd> to ($ExtIf) port smtp

I realized that using a tarpit discourages spammers, but if the primary
purpose is to simply protect your mail server, wouldn't the block
directive be more efficient?

Sure, but from what I understand the tarpit is cheap and the goal is to
consume spammers' or hijacked machines' CPU.